Light water nuclear power plants have on-site, underwater, spent-fuel storage pits, for the purpose of storing spent fuel (i.e., high burnup fuel) until the most intense heat and radioactivity have dissipated. Industry planners had anticipated that the spent fuel would then be transported to a reprocessing plant where unburned .sup.235 U and plutonium generated through neutron capture by .sup.238 U would be recovered and used to make new fuel.
However, in the late 1970's, two U.S. Government policies were initiated which necessitated changes in this process. The first policy prohibited the recovery of .sup.235 U and plutonium from the spent fuel. The result, according to subsequent analysis of the costs involved, was that the most economical concentration of .sup.235 U in fuel rods increased from about 3% to about 4.5% (all percentages herein are by weight). That is, natural uranium contains only 0.72% of the fissionable .sup.235 U isotope. The enrichment of natural uranium to higher percentages of the .sup.235 U isotope is necessary for the operation of light water reactors. With a nuclear power plant, the amount of energy one can extract from the fuel and convert to electricity increases with the amount of enrichment. However, enrichment is expensive, and costs increase as the percent enrichment increases. If the .sup.235 U and plutonium could be recovered from the spent fuel rods, these competing considerations result in a minimum cost at about 3% .sup.235 U in the fuel rods, which is the value used at present. However, if the valuable .sup.235 U and plutonium are not allowed to be recovered from the spent fuel, the minimum cost rises and is achieved at about 4.5% .sup.235 U in the nuclear fuel.
When the spent fuel is stored in the spent fuel pits, the Nuclear Regulatory Commission requires that the distance between the spent fuel rod bundles (fuel assemblies) be sufficient to prevent the initiation of a chain reaction. This distance is calculated as though the fuel were new since it has up to now been impractical to determine to what degree the fuel has been spent. Thus, if the percentage of .sup.235 U in the fuel is increased from 3% to about 4.5% to minimize costs under the new rules, the distance between the fuel assemblies in the spent fuel pits will need to be increased. But because the fuel assemblies are stored in racks made for 3% fuel, the 41/2% assemblies will need to be stored in every other space, a larger spacing than would be necessary to prevent a chain reaction even if the 41/2% fuel were new. Thus, the storage space in the fuel pits is considerably reduced.
The second new government policy prohibited the removal of the spent fuel assemblies from the pits. Thus, with the 41/2% fuel, not only will the distance between the assemblies be greater than for 3% fuel, but the accumulation of the spent fuel assemblies in the pits is rapidly depleting the storage space in the nuclear fuel plants.
This problem could be greatly alleviated if the amount of fissionable matter remaining in the fuel rods could be easily and accurately determined. If this could be accomplished, the fuel assemblies could be stored closer together in the pits as the distance between them could be based on the actual fissionable content in the rods rather than on the fissionable content that was present when they were new.